To the Editor: Dr Peralta and colleagues1 reported that “adding cystatin C to creatinine and albuminuria for risk prediction can more accurately reclassify persons and can distinguish important prognostic differences, namely a 3-fold risk of death and 4-fold risk of end-stage renal disease.” However, they did not explain why they chose cystatin C as a biomarker for CKD diagnosis or assess the accuracy of applying it.